It is already known, for example, to prepare polyisocyanates containing biuret groups from diisocyanates and water (U.S. Pat. No. 3,201,372); hydrogen sulphide (British Pat. No. 1,043,672); formic acid (U.S. Pat. No. 3,392,183) or tertiary alcohols (U.S. Pat. No. 3,358,010). In these processes, some of the isocyanate groups of the diisocyanates are converted into amino groups which then react with excess diisocyanate to form biuret polyisocyanates via the corresponding urea diisocyanates.
These prior art processes have various disadvantages. Firstly, the heterogeneous reaction of diisocyanates with water involves the risk of formation of insoluble polyureas which are difficult to remove. Furthermore, the above mentioned process invariably produce gaseous by-products such as carbon dioxide, carbon monoxide, carbon sulphoxide or olefines. Lastly, a particularly serious disadvantage of these prior art processes is that some of the isocyanate groups in the diisocyanate used as starting material are inevitably destroyed by amine formation. There have, therefore, been several attempts to provide a process for the production of polyisocyanates which contain biuret groups by the direct reaction of polyamines with polyisocyanates, without liberation of volatile by-products or destruction of isocyanate groups by amine formation. Because of the vigorous reaction of amino groups with isocyanate groups, however, considerable practical difficulties were encountered because of the very copious production of insoluble polyureas and crosslinked products. The only methods which met with some measure of success were, therefore, those in which very special starting materials were used. Thus, according to U.S. Pat. No. 3,441,588, higher molecular weight diaminopolyethers must be used, as the diamine component in order to prevent the formation of the above mentioned difficulty soluble by-products. It is clear that a process requiring the previous complicated preparation of diamino polyethers cannot be regarded as a commercially completely satisfactory solution to the problem. The process according to U.S. Pat. No. 3,824,266 is restricted to the use of diprimary aromatic diamines whose reactivity is reduced by steric or electronic effects and also does not provide a commercially acceptable method of producing polyisocyanates with biuret groups.
The process according to U.S. Pat. No. 3,903,126 also does not provide a commercially acceptable method of producing polyisocyanates with biuret groups by the direct reaction of organic polyisocyanates with simple aliphatic and/or cycloaliphatic polyamines. Thus, according to Example 16 of U.S. Pat. No. 3,903,126 preparation of polyisocyanates with biuret groups from hexamethylene diisocyanate and hexamethylene diamine requires reheating of the reaction mixture to about 180.degree. C. for about 12 hours in order to complete the reaction. Such long reheating at a high temperature is not only uneconomical, particularly under large scale production conditions, but also leads to discoloration of the reaction product so that its use in lightfast lacquers is very restricted. Moreover, when Example 16 of U.S. Pat. No. 3,903,126 was repeated, it was found that it was impossible by this method to obtain a biuret polyisocyanate which was both free from monomeric diisocyanate used as starting material and completely free from insoluble gel-like by-products.